Open top vessels such as induction furnaces used to remelt metals are operated so that the surface of metal during melting and the surface of the molten bath are exposed to ambient atmosphere. Air in the atmosphere tends to oxidize the melt causing loss of alloying additions, formation of slag causing difficulty in metal processing, shortening refractory life, promoting nonmetallic inclusions in the final casting, pickup of unwanted gases in the metals, porosity, and poor metal recovery. One solution to the problem is to enclose the induction furnace in a vacuum or atmosphere chamber for melting and/or processing of the metals. However, completely enclosed systems are very expensive and limit physical and visual access to the metals being melted.
Liquid fluxing salts, synthetic slag, charcoal covers and similar methods and compounds have historically been used in the high volume, cost-sensitive field of metal reprocessing for minimizing metal oxidation, gas pickup and loss of alloying additions. None of these techniques obstruct the required access to the metal surface but do necessitate additional handling and processing and cause disposal problems. All too frequently these top covers reduce furnace life or ladle refractory life, increase frequency of shutdowns for relining or patching of refractories and can produce non-metallic inclusions that have to be separated from the metal bath prior to pouring of the metal into a cast shape.
In searching for solutions to the above described problems, metallurgical industries turned to inert gas atmosphere blanketing for solutions to the problems. One group of gas blanketing systems is based on the principle of a gravitational dispersion of cryogenically-liquified inert gas over the surface of a hot metal to be blanketed. An example of cryogenic blanketing systems is disclosed and claimed in U.S. Pat. No. 4,990,183. While some cryogenic blanketing methods are found to be quite effective, their use is limited to those metallurgical facilities and vessels which can be supplied by well insulated cryogenic pipelines or equipped with cryogenic storage tanks in close proximity to the point of use of the liquid cryogen. This is not always practical, thus many cryogenic blanketing systems have been plagued by poor efficiency due to the premature boil-off of the cryogenic liquid and oversimplified design of dispersing nozzles that wasted the boiled-off gas phase. Moreover, existing cryogenic dispensers usually fail to uniformly disperse the cryogenic liquid over the blanketed surface leading to a transient accumulation or entrapment of the liquid in pockets under the slag or dross which may sometimes result in explosions in a subsequent rapid boil-off.
Another group of gas blanketing systems is based on the principle of shrouding molten metal surfaces with a curtain or barrier of substantially inert gases at room temperature discharged from multiple nozzles, diffusing plates or tubes where the gas is discharged parallel to the blanketed surface. Such techniques are shown in U.S. Pat. Nos. 4,823,680 and 5,195,888. Because of the required geometric configuration the horizontal barrier systems shown in the prior art are placed at the top plate of a furnace housing thus causing the furnace operator to eliminate normal fume venting devices. Since the diffusing elements are nozzles that expel inert gas across a large portion of the top of the furnace and are spaced a distance from the melt surface, large volumes of gases must be consumed to significantly reduce oxygen (and/or nitrogen) concentrations inside of the furnace crucible. If the diffusing elements were moved closer to the rim of the furnace crucible, they would suffer clogging from metal splash because of the small diameter of the gas orifices, impact from heat from the molten metal, or the induction field in an induction furnace. They could also be mechanically damaged during charging of solids into the crucible. Thus there is a need for a blanketing method that will avoid the problems with the prior art methods and devices.